In recent years, deterioration of concrete structures in sewerage systems or the like in Japan has been reported frequently. Deterioration occurs not only in Japan but also in other countries and has been reported in Australia, Egypt, South Africa, the U.S., and the like. Because it costs a large amount of money to build sewerage systems or the like, it is important to take appropriate measures against the deterioration of concrete so as to enable the system effectively to function over a long time.
It is known that the deterioration of concrete is caused by two types of microorganisms, i.e., sulfate reducing bacteria and sulfur oxidizing bacteria such as the genus Thiobacillus or the like. In the process of the deterioration of concrete by these microorganisms, firstly, sulfate in sewage (normally, the concentration of sulfate in sewage is within the range of 20 to 40 mg/l) is reduced by the sulfate reducing bacteria under anaerobic conditions and hydrogen sulfide is thereby generated. Next, the hydrogen sulfide is absorbed by water on the concrete wall surface and is oxidized by sulfur oxidizing bacteria under aerobic conditions. Sulfuric acid is thereby generated. Calcium in the concrete is changed to calcium sulfate (plaster) by the generated sulfuric acid. In this way, the concrete becomes fragile (deteriorates).
Among the aforementioned two types of microorganisms, the sulfur oxidizing bacteria is considered to be the main cause of the deterioration of concrete. Various methods have been proposed for inhibiting the change of hydrogen sulfide to sulfuric acid by the sulfur oxidizing bacteria.
One of these methods is a method of reducing the concentration of hydrogen sulfide which is a substrate for sulfur oxidizing bacteria. For example, methods are known in which air or oxygen is injected into sewage to oxidize hydrogen sulfide before the hydrogen sulfide is absorbed by water on the concrete wall surface, and to suppress the activity of the anaerobic sulfate reducing bacteria so that the generation of hydrogen sulfide is reduced. Among these methods, the method of injecting air into the sewage is a relatively simple method. However, in this method, the injected air may affect the equilibrium between the gas and liquid of the hydrogen sulfide such that the amount of hydrogen sulfide diffused in the air is larger than that in a case in which air is not injected. Further, hydrogen sulfide can be oxidized more effectively in the method of injecting oxygen into the sewage than in the method of injecting air. However, there is a drawback in that the cost of the method is high.
Another inhibiting method is a method of adding a large amount of chlorine, hydrogen peroxide, potassium permanganate, or a metallic salt whose metal is iron, zinc, lead, copper, or the like into sewage so that the hydrogen sulfide within the sewage bonds therewith. Further, Japanese Patent Application Laid-Open (JP-A) No. 7-70561 discloses a method of inhibiting the deterioration of concrete in which a water-soluble quinone derivative is added into sewage to oxidize hydrogen sulfide and to suppress the activity of the anaerobic sulfate reducing bacteria. However, since the material added into the sewage disappears as it flows, these methods are not effective and are expensive.
Moreover, a method of inhibiting the deterioration of concrete is known in which sulfur oxidizing bacteria is killed by mixing an antibacterial agent, which is an organic compound, with concrete. However, the antibacterial agent may generate pinholes or cracks in the concrete and may reduce the durability of the concrete. Further, at present, the use of Na--PCP (sodium pentachlorophenol) having a strong antibacterial action is prohibited.
Further, it is known that the growth of sulfur oxidizing bacteria is inhibited by a metal ion. JP-A No. 4-149053 discloses a method of inhibiting the deterioration of concrete in which a metal (e.g., copper, nickel, tin, lead or the like which is difficult to dissolve in water and is soluble in sulfuric acid) or the oxide of such a metal is added to concrete. In this method, metal ion is eluted from the metal and/or the metal oxide by sulfuric acid generated by sulfur oxidizing bacteria and inhibits and/or kills the sulfur oxidizing bacteria. However, in this method, because a metal having high solubility in sulfuric acid, or a metallic oxide having high solubility in sulfuric acid, or a mixture thereof is used, in order to prevent the deterioration of concrete over a long time, a large amount of metal or the like must be used. Further, since the heavy metal ion of nickel, tin, lead or the like is eluted into the sewage, water pollution may be caused by these metals.
Furthermore, JP-A No. 6-16460 and JP-A No. 6-16461 disclose methods of inhibiting the deterioration of concrete in which a metal complex such as nickelocene or nickeldimethylglyoxime is added to concrete. However, these metal complexes are carcinogenic and cause problems in terms of safety.
Several methods which use a material having resistance to sulfuric acid have also been proposed as methods of inhibiting the deterioration of concrete. For example, JP-A No. 63-16072 and JP-A No. 2-265708 disclose methods of protecting concrete by the lining of an epoxy resin, a polyester resin, or the like. Moreover, JP-A No. 1-55493 discloses a method of protecting concrete by the lining of a glass material.
However, in these methods, the construction cost is high, and since the lined material is peeled through pinholes, the life of concrete is short. Therefore, in order to maintain the effect of inhibiting deterioration, it is necessary to recoat the concrete periodically. The operation of the sewerage system or the like has to be stopped for a long time each time the concrete is coated.
Still further, a method in which concrete having excellent acid resistance and sulfate resistance is obtained by using slag cement or the like having a high percentage content of sulfate is also known. However, the strength of this slag cement is poor and the deterioration of the obtained concrete due to sulfuric acid cannot be completely prevented.